Radio communication systems rely upon transmitters to send modulated radio frequency signals and receivers to process the transmitted radio frequency signals. The processing of radio frequency signals commonly entails converting the relatively high frequency incoming signal to a relatively low-frequency signal, which is then demodulated to extract the useful information in the originally transmitted signal. The frequency conversion process typically introduces low-frequency distortion, commonly in the form of a dc offset signal, into the frequency converted signal. The present invention is directed toward reducing low-frequency distortion of this type.
Most receivers today. use the superheterodyne architecture, which is referred to as a single-conversion receiver, double-conversion receiver, etc., as a function of how many frequency conversions the input signal goes through before being demodulated. Typically, the demodulation is done at some convenient carrier frequency, also known as an intermediate frequency, or IF, which is low enough to allow easy amplification but high enough to accommodate the modulation bandwidth.
The "direct-conversion receiver" or "zero-IF receiver" is used to convert a signal to a very low-frequency (around zero) and perform demodulation at baseband frequencies. The main obstacle in using this architecture for high-performance receiver designs- is that the conversion to baseband frequencies always introduces DC offsets which are in general hard to eliminate if the useful signal has spectral components at zero frequency.
Solutions to this problem exist for specific modulation types, but not for the general case. One specific solution is to use AC coupling after the last frequency conversion. This solution works well for modulation methods that have a spectrum gap at the carrier frequency.
Another proposed solution described in U.S. Pat. Nos. 5,086,437 and 5,263,194 takes advantage of the idle receive time in systems with intermittent transmission in order to store the DC offset present in the absence of the input signal and then subtract the stored value when receiving the input signal.
U.S. Pat. Nos. 4,926,443; 5,241,702; and 5,442,655 describe techniques that use special properties of the modulated carrier in order to compute the added DC offset error. Spread-spectrum methods have also been used to solve the DC offset problem, as described in U.S. Pat. No. 4,736,390.
The foregoing prior art approaches solve particular problems, but are not generally applicable to all direct conversion receivers. Moreover, these prior art approaches do not provide a simple implementation that can be readily incorporated into old and new direct conversion receiver designs.